1. Field of the Invention
The present invention relates to a process for surface treatment of vulcanized rubber and a process for production of rubber-based composite material.
2. Prior Art
Surface treatment or surface modification is commonly used in various fields to make a substrate hydrophilic, water-repellent, or adherent. For example, surface treatment is performed on vulcanized rubber to be coated with a paint or to be bonded to any other material such as rubber, metal, and plastics for the production of composite materials. There are several kinds of surface treatments.
A known process for surface treatment of vulcanized rubber to be made adherent consists of intensively oxidizing the surface of vulcanized rubber with a strong acid or a strong oxidizing agent, thereby making minute cracks over the entire surface. This process suffers the disadvantage that a strong acid or strong oxidizing agent needs a great care for its handling and remarkably damages the surface of vulcanized rubber. Moreover, it does not make vulcanized rubber adherent sufficiently.
There are other known processes for surface treatment, which consist of exposing vulcanized rubber to chlorine gas, dipping vulcanized rubber in chlorine gas-bubbling water, or treating vulcanized rubber with a halogen compound. (See Japanese Patent Publication No. 36910/1977.) These processes are based on the principle that chlorine attacks double bonds in rubber to form chlorine-containing groups required for adhesion. The disadvantage is that the treated surface is resinified when treated rubber (NR or SBR) is combined with other material (such as metal and resin) to give rubber vibration isolators. The resinified surface poses a problem with poor adhesion and poor heat resistance. Another disadvantage is the yellowing of the treated surface. For example, yellowing occurs when the above-mentioned surface treatment is applied to the golf ball cover of vulcanized rubber whose main component is balata (trans-polyisoprene). This yellowing adversely affects the appearance of the coated golf ball. In addition, treatment with chlorine gas or halogen compound involves the possibility of causing an environmental problem.
There is another process for surface treatment of vulcanized rubber which is designed to activate the surface by etching with low-pressure glow plasma in oxygen or a mixture of oxygen and CF.sub.4. This process is capable of uniform surface treatment. However, it needs a large vacuum equipment (for 10 Torr or below) when it is put to industrial use and it also needs a great installation cost and running cost for continuous operation. In addition, treatment with plasma may not produce the desired effect because oil and water are released from the surface of vulcanized rubber in the atmosphere of reduced pressure. Moreover, plasma is liable to generate heat during treatment, and hence it cannot be readily applied to a substrate having a low melting point.
The same problem as mentioned above is also involved in the industrially established corona treatment.
In order to address the above-mentioned problems, the present applicant proposed in Japanese Patent Laid-open No. 202208/1993 corresponding to U.S. Pat. No. 5,316,739 a process for treating the surface of vulcanized rubber with plasma of a gas of halogen-containing molecules or oxygen-containing molecules under atmospheric pressure. Requiring no solvents, this process is capable of surface treatment in a simple manner without environmental pollution. In addition, treatment by this process makes the surface more adherent than low-pressure glow plasma treatment by the conventional process. Another advantage of this process is that treatment only affects the thin surface layer, leaving the bulk of vulcanized rubber intact.
However, the plasma treatment under atmospheric pressure suffers the disadvantage of requiring a large amount of helium to dilute the treating gas for stable discharging. In addition, the plasma treatment needs a special power unit and precludes using the general-purpose power unit for corona treatment or low-pressure plasma treatment because its impedance at the time of discharge is lower than that of corona treatment and higher than that of low-pressure plasma treatment. These disadvantages lead to a high treatment cost.